1. Field of the Invention
The present invention relates generally to nuclear reactor power plants or facilities, and more particularly to a new and improved mechanical drive system for moving alternate nuclear reactor fuel rods within the nuclear reactor core fuel assemblies so as to optimize excess or free neutron utilization in the economically useful production of additional nuclear reactor plutonium fuel, in lieu of permitting a substantial proportion of such excess or free neutrons to be parasitically absorbed by means of the nuclear reactor core neutron absorbing materials, such as, for example, soluble boron poisons dissolved within the core coolant, whereby the boron transmutes into various isotopes which are economically useless in connection with the operation of the nuclear reactor facility or power plant.
2. Description of the Prior Art
In conventional nuclear reactor power plants or facilities, the fuel disposed within the nuclear reactor fuel rods is usually in the form of cylindrical pellets of uranium dioxide (UO.sub.2), and the uranium component is enriched uranium as opposed to natural uranium. Enriched uranium has a substantially higher proportion of U-235, and a correspondingly lower proportion of U-238, contained therein than is characteristic of natural uranium, and is therefore considerably more expensive than natural uranium. Nuclear reactor power plants or facilities cannot, however, utilize nuclear fuel which is entirely comprised of natural uranium in view of the fact that natural uranium does not exhibit sufficient reactivity as to render the reactor, and the fission reactions or processes thereof, critical. A reactor is considered to be critical when each fission reaction or process leads to or produces one other fission process or reaction. Correspondingly, a reactor is considered to be sub-critical when, on the average, each fission process or reaction leads to or produces less than one further or additional fission reaction or process, and similarly, the reactor is considered to be super-critical when each fission reaction or process, on the average, leads to or produces more than one further or additional fission reaction or process.
Reactivity is a measure of the number of excess or free neutrons which are produced by means of the reactor fission reactions or processes and which are therefore available for use in the production or generation of subsequent or additional fission reactions or processes, it being appreciated that at least one excess or free neutron is required for an additional or subsequent fission process or reaction to be achieved in order to sustain the reactor's fission chain reaction and to maintain the reactor critical. Reactivity may therefore be considered to be either positive, zero, or negative, zero reactivity corresponding to a critical power state within the reactor. In order to therefore provide the reactor with the longest possible fuel life cycle, conventional reactors employ only enriched uranium fuel within the reactor fuel rods in view of the fact that enriched uranium exhibits the highest possible degree or level of reactivity, which high degree or level of reactivity becomes increasingly more important as the particular fuel cycle proceeds. More particularly, a better appreciation for the operation of a commercial nuclear reactor power plant or facility may be attained from the broad conceptual axiom that the reactor will in fact exhibit a greater degree or level of reactivity at the beginning of a fuel cycle than in the middle of a fuel cycle or at the end of a fuel cycle in view of the fact that reactivity is dependent upon, or is a function of, the amount of fuel within the core. The reactivity of the reactor core therefore progressively declines from the beginning of the fuel cycle to the end of the fuel cycle, wherein the end of the fuel cycle is determined by that point at which the reactor exhibits insufficient reactivity to maintain the fission chain reaction.
Reactivity control within a typical nuclear reactor power plant or facility is conventionally accomplished through several different means, such as, for example, neutron absorbing control rods, burnable poisons, and poisons dissolved within the reactor coolant, with the former two means being conventionally utilized for incremental or stepwise and continuous reactor power adjustment functions or operations, respectively, while the latter means is conventionally employed for true continuous reactivity control over the fission processes or reactions throughout the reactor in a uniform manner in view of the fact that the soluble poisons, such as, for example, boron, are disposed within the core coolant which is present throughout the core as opposed to the specific or concentrated locations of the control rods and burnable poisons.
In view of the foregoing operational characteristics of the reactor to the effect that the reactivity thereof is substantially greater at the beginning of the fuel cycle than in the middle of the fuel cycle, or at the end of the fuel cycle, it naturally follows that substantial reactivity control must be exercised in the beginning stages of the fuel cycle in order to effectively restrain the core reactivity, whereas such reactivity controls must be relaxed as the fuel cycle proceeds, and particularly as the fuel cycle approaches its end, in order to effectively permit the core to exhibit sufficient reactivity so as to sustain the reactor's fission chain reaction. This difference in reactivity control function or operational levels is achieved by varying the percentage amount of the soluble boron poisons within the reactor core coolant. In particular, as the fuel cycle proceeds, the amount of soluble boron poisons present within the reactor core coolant is progressively reduced, or in other words, a predetermined percentage amount of the poisons is continuously removed from the core coolant.
In the performance of the reactivity control functions or operations by means of the soluble boron poisons, it is of course appreciated that the poisons parasitically absorb the excess neutrons whereby the boron is transmuted into various isotopes. While this mode of operation is therefore certainly one means of effectively controlling the reactivity of the nuclear reactor throughout its core fuel life cycle, the neutron-absorbing poisons absorb neutrons and effectively remove them from the core in an essentially wasteful manner in that the neutrons are effectively consumed without producing any useful product, such as, for example, additional plutonium fuel. Consequently, the conventional mode of operation of the nuclear reactor power plant or facility is seen to comprise a relatively inefficient depletion or consumption of the uranium fuel which, viewed from a different perspective, effectively results in greater fuel costs than could otherwise be achieved. If in lieu of the conventional consumption of the reactor core fuel, and the reactivity control operatively associated therewith, means could be developed wherein the excess neutrons were effectively utilized in the useful production of additional plutonium fuel, as opposed to simply being absorbed by the boron poisons so as to result in the economically useless production of various isotopes, the reactivity level of the reactor core would be enhanced and the fuel cycle life of the reactor core would be significantly extended resulting in considerably lower fuel costs.
Accordingly, it is an object of the present invention to provide new and improved means for optimizing utilization of free or excess neutrons generated within the reactor core by means of the fission processes or reactions.
Another object of the present invention is to provide new and improved means for optimizing utilization of excess or free neutrons generated within the reactor core by means of the fission processes or reactions so as to achieve reactivity control of the reactor core in an economically enhanced manner.
Yet another object of the present invention is to provide new and improved means for optimizing utilization of free or excess neutrons generated within the reactor core by means of the fission processes or reactions so as to produce additional plutonium fuel.
Still another object of the present invention is to provide new and improved means for optimizing utilization of free or excess neutrons generated within the reactor core by means of the fission processes or reactions so as to substantially extend the fuel cycle or fuel service life of the reactor core.
Yet still another object of the present invention is to provide new and improved means for optimizing utilization of free or excess neutrons generated within the reactor core by means of the fission reactions or processes so as to achieve maximization of fuel cycle cost benefits.
Still yet another object of the present invention is to provide new and improved means for optimizing utilization of free or excess neutrons generated within the reactor core by means of the fission processes or reactions wherein the new and improved means of the present invention is applicable to existing power plants and facilities whereby the same may be retrofitted with the system of the present invention.